1. Field of the Invention
The present invention relates to water-soluble quaternary ammonium vinyl monomers, to cationic water-soluble homopolymers and copolymers of these monomers with acrylamide and other compounds, to a vinyl compound specially useful for production of the aforesaid quaternary ammonium monomers, to paper of improved dry strength resulting from a content of the aforesaid copolymers, and to the various methods involved in making the foregoing products.
Dry strength paper is a specialty of the papermaker's art. It is paper which possesses excellent tensile strength when dry but which possesses substantially no tensile strength when wet. Dry strength paper rapidly disintegrates when allowed to fall into a natural environment and so solves an ecological problem.
There is also a demand for a cationic polymer which provides only dry strength, but which can be made to provide wet strength as well by an inexpensive and easy modification.
At the present time, dry strength paper is manufactured on a large commercial scale by adding a water-soluble anionic polymer to an aqueous suspension of cellulose papermaking fibers, precipitating the polymer on the fibers by the action of alum, forming the fibers into a web and drying the web; cf. Canadian Pat. No. 477,265. A disadvantage of the process is that the papermaking system throughout the operation is at an acid pH with resulting corrosion of the papermaking machine. A further disadvantage is that the paper which it produces is acid and undergoes acid tendering as it ages. An additional disadvantage is that the sulfate ions (introduced as part of the alum) are not adsorbed by the cellulose and so accumulate in the aqueous phase of the system, creating a disposal problem. Moreover, anionic polymers are poorly effective in papermaking systems which contain black liquor solids, so that for best results the pulps must be well washed. Water of sufficient purity for pulp washing is now a critical commodity in many paper mills.
2. Description of the Prior Art
It has long been known that the water-soluble vinyl cationic polymers which are composed of acrylamide units in predominant proportion and of vinyl quaternary ammonium units in minor proportion are good dry strengthening agents when added to the aqueous cellulose fiber suspensions from which paper is made (cf. Wilson et al. U.S. Pat. Nos. 2,884,057; 2,884,058; and Moore, U.S. Pat. No. 3,077,430) even in the absence of alum, at alkaline pH values up to at least pH 10; that they are effective at acid pH values at least down to pH 4; and that they are not sensitive to dissolved sulfate ions (at least in moderate amount). These polymers therefore possess major practical advantages.
A disadvantage of the polymers of the aforesaid patents is that they are difficult and costly to make, so that they are uneconomic. Accordingly, attempts to introduce these polymers into the commercial manufacture of paper have not proved successful.